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調用時發生的NullPointerException異常getPlaylist（）方法來自Android中的其他活動。 的方法應該接受，則音頻信息使用光標光標返回它= getContentResolver（）查詢（MediaStore ... ）當調用具有getContentResolver（）的方法時的NPE Android中的查詢
下面是詳細信息。
我正在使用兩個活動（AudioPlayerActivity，FileManager）的自定義音頻播放器。 AudioPlayerActivity使用FileManager **獲取音頻文件列表，然後播放它們。
這裏是他們的片段。以下是AudioPlayerActivity.java
ArrayList<HashMap<String, String>> audioList = new ArrayList<HashMap<String, String>>();
mp = new MediaPlayer();
fileManager = new FileManager();
audioList = fileManager.getPlaylist(); // NPE when calling this method!
playAudio(0);
，下一個是FileManager.java，其內容爲所有音頻文件，並使用MediaStore API存儲在ArrayList中的詳細信息（如MP3 ID標籤）。
public class FileManager extends Activity {
@Override
public void onCreate(Bundle SavedInstanceState) {
super.onCreate(savedInstanceState);
// no view. just getting audio files
}
public ArrayList<HashMap<String, String> getPlaylist() {
String[] proj = { MediaStore.Audio.Media._ID };
Cursor cursor = getContentResolver().query(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, proj, null, null, null}
Log.d("debug", "is cursor?"); // NPE here!
return list;
}
logcat的
09-30 11:55:35.265: D/AndroidRuntime(13516): Shutting down VM
09-30 11:55:35.265: W/dalvikvm(13516): threadid=1: thread exiting with uncaught exception (group=0x41d37c08)
09-30 11:55:35.270: E/AndroidRuntime(13516): FATAL EXCEPTION: main
09-30 11:55:35.270: E/AndroidRuntime(13516): Process: project.sample.audioplayer, PID: 13516
09-30 11:55:35.270: E/AndroidRuntime(13516): java.lang.RuntimeException: Unable to start activity ComponentInfo{project.sample.audioplayer/project.sample.audioplayer.AudioPlayerActivity}: java.lang.NullPointerException
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread.access$900(ActivityThread.java:161)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.os.Handler.dispatchMessage(Handler.java:102)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.os.Looper.loop(Looper.java:157)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread.main(ActivityThread.java:5356)
09-30 11:55:35.270: E/AndroidRuntime(13516): at java.lang.reflect.Method.invokeNative(Native Method)
09-30 11:55:35.270: E/AndroidRuntime(13516): at java.lang.reflect.Method.invoke(Method.java:515)
09-30 11:55:35.270: E/AndroidRuntime(13516): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
09-30 11:55:35.270: E/AndroidRuntime(13516): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
09-30 11:55:35.270: E/AndroidRuntime(13516): at dalvik.system.NativeStart.main(Native Method)
09-30 11:55:35.270: E/AndroidRuntime(13516): Caused by: java.lang.NullPointerException
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.content.ContextWrapper.getContentResolver(ContextWrapper.java:104)
09-30 11:55:35.270: E/AndroidRuntime(13516): at project.sample.audioplayer.FileManager.getPlaylist(FileManager.java:35)
09-30 11:55:35.270: E/AndroidRuntime(13516): at project.sample.audioplayer.AudioPlayerActivity.onCreate(AudioPlayerActivity.java:70)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.Activity.performCreate(Activity.java:5426)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
09-30 11:55:35.270: E/AndroidRuntime(13516): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
09-30 11:55:35.270: E/AndroidRuntime(13516): ... 11 more
它給的NullPointerException在線路具有getContentResolver（）。查詢（）。
我錯過了什麼？或者是不可能在方法中使用Cursor cursor = getContentResolver（）。query（MediaStore ...）而不是在onCreate（）方法？
我敢肯定，AudioPlayerAcitivity.java是沒有問題的，因爲它使用文件類，而不是MediaStore在FileManger.java時工作。
後從logcat的一個完整的堆棧跟蹤 – Karakuri 2014-09-30 02:52:27